The present invention relates to a brake booster with position-dependent pressure equalization for a motor vehicle brake system, comprising a force input element that is or can be connected to a brake pedal, a chamber arrangement with a working chamber and a vacuum chamber that are separated from one another by a movable wall, and a control device with a control valve, by means of which the working chamber can be optionally connected to the vacuum chamber or to the surrounding atmosphere as a result of a displacement of the force input element, wherein the control valve has a control valve housing, which is coupled to the movable wall and in which the force input element is guided in a displaceable manner against the action of a return spring together with a control piston, and wherein the control piston further comprises a valve element that is optionally prestressed into sealed abutment on a first valve seat provided on the control valve housing or into sealed abutment on a second valve seat provided on the control piston of the control valve by means of a tension spring.
Such brake boosters are prior art. Thus, for example the document DE 42 27 879 A1, and corresponding U.S. Pat. No. 5,546,846 A, both of which are incorporated by reference herein in entirety, discloses such a brake booster. With this in a known manner an actuating force exerted via a force input element is transmitted to a control piston, on which a sealing seat between the working chamber and the surrounding atmosphere is mounted. As a result of actuation of the force input element and, with it, the control piston this sealing seat is lifted off the valve element, thereby resulting in a fluid connection between the surrounding atmosphere and the working chamber. An excess pressure accordingly builds up in the working chamber compared to the vacuum chamber, acts upon the movable wall and in order to assist the actuating force displaces the control valve housing until a state of equilibrium develops in the advanced position of the control valve housing and both valve seats move back into abutment on the valve element. Once the force input element is released by reducing a pedal actuating force, the valve element is pushed back by the control piston under the action of a return spring, so that the second sealing seat between working chamber and surrounding atmosphere opens and a pressure equalization may occur at the movable wall. As soon as the pressure equalization has occurred, the initial state is restored. In order to guarantee a reliable mode of operation, the valve element is prestressed by means of a tension spring in such a way that its sealing surface is biased towards the two valve seats. This means however that for the previously described return movement after release of the brake pedal the return spring has to exert a high enough force to overcome also the prestressing force of the tension spring at the valve element that counteracts this force.
It has been shown in the past and was accordingly realized in the previously cited document DE 42 27 879 A1 that it may be advantageous if the valve element is relieved from pressure. By means of such a pressure relief the effect may be achieved that the response behaviour but also the return movement are not hindered by unwanted differential pressures at the valve element. For this reason the previously cited document DE 42 27 879 A1 provides that in the valve element a plurality of pressure-equalizing openings are provided, which enable a permanent pressure equalization between both sides of the valve element.
Similar solutions are to be found in a large number of documents relating to the background art, such as for example in the document DE 44 41 149 A1, and corresponding U.S. Pat. No. 7,066,073 B1, both of which are incorporated by reference herein in entirety, the document EP 0 802 870 B1, and corresponding U.S. Pat. No. 5,657,680 A, both of which are incorporated by reference herein in entirety, and the document EP 0 950 595 B1. The document U.S. Pat. No. 5,564,325 also discloses a form of construction, in which a permanent pressure equalization at the valve element occurs by means of a connection opening.